The present invention relates to narrowband filters and more particular to narrowband filters suited for use in a system where a digital word describing characteristics of the filtered signal is desired. Present analog filtering techniques utilize both active and passive components to yield continuous time filtering. The signal output from the filter is then detected by suitable means to yield magnitude information. This magnitude information is then converted to a digital word by means of an analog to digital converter. Such filters do not have high center frequency stability necessary in high Q applications. Present digital filtering techniques employ signal sampling coupled with numerical computation capability like that found in minicomputers to yield information concerning spectral content. Such techniques require high timing accuracy and high computational speed in the processor which leads, consequently, to complex systems.